1. Field of the Invention
This invention relates to a process for the production of red to yellow nitride or oxidenitride based pigments, in particular nitrides and oxidenitrides containing tantalum, by nitriding nitridable metal compounds with flowing ammonia at 700 to 1250xc2x0 C. The nitrides and oxidenitrides are produced in pigment quality on an industrial scale.
2. Description of the Related Art
Coloured pigments are subject to differing considerations depending upon the type of application and the use of the articles coloured therewith. Toxicologically questionable constituents may be released from articles coloured with pigments based on oxide, sulfide or selenide heavy metal compounds or decorated using such pigments on contact with acidic or alkaline solutions, for example nickel, cobalt or chromium may be released from spinels, cadmium from cadmium sulfide yellow and cadmium and selenium from cadmium sulfoselenide red or orange. Another problem is the release of toxic heavy metals when articles coloured in this manner are landfilled or incinerated in waste incinerators.
The stated problems may be reduced or completely eliminated by using nitride or oxidenitride pigments based on innocuous metals, in particular nitrides and oxidenitrides containing tantalum. Such pigments in particular cover the yellow to red range of the spectrum.
Tantalum(V) nitride (Ta3N5), c.f. US-A 5,569,322, and oxidenitrides having a perovskite, spinel, pyrochlore and elpasolite structure, including pigments containing tantalum, c.f. EP-A 0 697 373, are known for colouring plastics, paints and for the production of stovable decorative colours and glazes. Such pigments may be obtained by nitriding suitable metal compounds, such as oxides, oxide hydrates, nitrates, carbonates, oxalates and halides, with ammonia at 700 to 1250xc2x0 C. Hitherto known nitriding processes are not suitable for obtaining nitrides and oxidenitrides on an industrial scale because nitriding is incomplete and the colour tone and/or brightness are unsatisfactory from a coloristic viewpoint and/or because scaling up previous processes from the laboratory scale to the industrial scale would entail highly complex plant and equipment.
In the hitherto conventional nitriding process according to US-A 5,569,322 and EP-A 0 697 373, the powder or powder mixture to be nitrided is nitrided in corundum boats by passing ammonia over it in an externally heated tubular reactor. While the duration and/or temperature of nitriding may indeed be reduced by using mineralisers, for many applications, the mineralisers must be leached back out of the nitriding product. The process according to EP-A 0 184 951, in accordance with which oxidenitrides having a perovskite structure are nitrided by nitriding nitride, oxidenitride or oxide metal compounds with ammonia as the nitriding and reducing agent, also gives no indication of how the laboratory process may most conveniently be scaled up to industrial scale.
According to DE-A 34 43 622, titanium dioxide powder may be converted on an industrial scale into black titanium nitride powder by means of nascent nitrogen at 800xc2x0 C. The source of the nascent nitrogen is ammonia, which is passed at a velocity of 3 cm/sec through the tower reactor equipped with stirrer blades. In this reaction, Ti4+ is at least partially converted into Ti3+ and red to yellow pigments are thus not obtained. Tantalum(V) nitride may be produced according to US-A 2,461,020 in a stirred reactor. In this case, a tantalum compound, such as Ta2O5, is reacted in an ammonia atmosphere in the presence of a readily oxidisable metal such as magnesium. The resultant magnesium oxide must be removed from the reaction product if the reaction product is to be usable as a pigment.
According to GB 979,277, nitrides of boron, aluminium and gallium may be obtained from the corresponding phosphides or arsenides with ammonia at 700 to 1200xc2x0 C. in a fluidised bed. No other starting materials are mentioned. It is known from DE-PS 369 298 and EP 0 571 251 A1 to use a rotary kiln for nitriding reactions.
The object of the invention is to provide a process which may be performed on an industrial scale for the production of pigments of the stated type by nitriding suitable metal compounds with ammonia at 700 to 1250xc2x0 C. The process should be simple to handle and give rise to homogeneous products.
This object is achieved by the process according to the main claim. A process has been found for the production of a red to yellow pigment based on a nitride or oxidenitride with at least one metal of a valence state in the range from +3 to +5, comprising nitriding of a pulverulent metal compound or of a mixture of metal compounds from the series of oxides, oxide hydrates, mixed oxides, carbonates, nitrates, oxidenitrides having a lower degree of nitriding than an oxidenitride to be produced, halides, oxidehalides and nitridehalides, wherein the oxygen of an oxidenitride to be produced originates from the metal compound(s) used, with ammonia flowing over the metal compound(s) at 700 to 1250xc2x0 C., which process is characterised in that nitriding is performed in a rotary tube or fluidised bed reactor in the presence of at least one pulverulent oxide uniformly dispersed in the metal compound(s) to be nitrided and selected from the series SiO2, GeO2, SnO2, TiO2, ZrO2 and HfO2, wherein the selected nitriding temperature substantially excludes nitriding of the stated oxides of the tetravalent metals.
Although the metal compound(s) to be nitrided may be nitrided directly in a rotary tube or fluidised bed reactor, an oxide from the series SiO2, GeO2, SnO2, ZrO2 and HfO2 or a mixture of such oxides is conveniently added as a diluent during nitriding of the metal compound(s) to be nitrided; the added oxides should themselves substantially not be nitrided under the nitriding conditions. The addition of one or more of the stated oxides, which may be added in a quantity of preferably 1 to 20 wt.%, favours nitriding and reduces any sintering of the pulverulent reaction mixture, which impairs nitriding. In this manner, homogeneous products are obtained. The process according to the invention is particularly suitable for nitriding a powder mixture containing tantalum.
Rotary tube reactors and fluidised bed reactors are known per se in specialist circles, but they have never hitherto been considered for the production of yellow to red nitride or oxidenitride based pigments. In rotary tube reactors, which are preferred according to the invention, the particles to be nitrided are constantly being uncovered, such that homogenous products may be obtained. Before nitriding, powder mixtures are conveniently homogenised by means of an intensive mill, such as a ball mill, such that the individual constituents are in intimate contact with each other and segregation is avoided in the rotary tube reactor. In this manner, oxidenitrides having at least two different metal atoms may be produced from two metal compounds. Preferably, however, uniform substances are introduced into the rotary tube reactor, for example spray-dried powders produced from two or more metal compounds, and mixed oxides or xerogel powders produced by a sol/gel process. The process according to the invention gives rise to a higher space/time yield than has been possible in hitherto known processes for the production of red to yellow pigments of the stated type.
Fluidised bed reactors ensure undisturbed contact between the particulate educt to be nitrided and ammonia, but the particle fineness conventional for pigment purposes entails downstream apparatus for separating and recirculating dusts.
The educt in fluidised bed nitriding should be uniform, i.e., where oxidenitrides having more than one metal atom are to be produced, the individual particles contain all the metal atoms in the suitable atomic ratio.
Nitriding may be performed using ammonia or a dried gas containing ammonia, with ammonia or an ammonia/nitrogen gas mixture containing at least 90% ammonia preferably being used. During nitriding, ammonia or a nitriding gas containing ammonia is passed through the reactor. When ammonia is used, the flow velocity is at least 0.5 cm/sec, preferably at least 1 cm/sec; a flow velocity of 2 to 3 cm/sec is particularly preferred.
The person skilled in the art will adjust the nitriding temperature to the starting metal compounds used. A preferred temperature range is from 800 to 1000xc2x0 C. Long reaction times are frequently necessary at lower temperatures. At higher temperatures, there is a risk of the nitride dissociating into metal and nitrogen, undesirably reducing the quality of the resultant nitride or oxidenitride pigment.
Nitride or oxidenitride based red to yellow pigments in particular have a Ta3N5, spinel, perovskite, elpasolite or pyrochlore structure, but other crystal forms are not excluded. The process of the invention is in particular suitable for the production of red tantalum(V) nitride and red-orange oxidenitrides of the formula Ta3xe2x88x92xZrxN5xe2x88x92xOx, in which x is a number greater than 0 up to the saturation limit of the Ta3N5 lattice. The stated oxidenitrides are obtainable from (Ta,Zr) xerogels according to J. Mater. Chem. 1994, 4(8), 1293-1301; the saturation limit is at approx. x=0.6.
The process is also suitable for the production of oxidenitrides, as described in EP-A 0 697 373. Examples of these are:
a) coloured pigments based on oxidenitrides of perovskite structure, the nitrogen to oxygen atomic ratio of which determines the colour, having the general formulae
A1xe2x88x92uAxe2x80x2uB O2xe2x88x92uN1+u or
Axe2x80x2B1xe2x88x92wBxe2x80x2wO1+wN2xe2x88x92w 
xe2x80x83in which A, Axe2x80x2, B and Bxe2x80x2 denote one or more cations from the series
A: Mg2+ [sic], Ca2+, Sr2+, Ba2+
Axe2x80x2: Ln3+ (rare earth metal), Bi3+, Al3+, Fe3+
B: V5+, Nb5+, Ta5+,
Bxe2x80x2: Ti4+, Zr4+, Hf4+, Sn4+, Ge4+ and
u and w denote a number in the range from 0 to 1;
b) oxidenitrides crystallised in the pyrochlore structure of the general formulae
AxAxe2x80x22xe2x88x92xB2O5+xN2xe2x88x92x or
Axe2x80x22B2xe2x88x92yBxe2x80x2yO5+yN2xe2x88x92y 
xe2x80x83in which A, Axe2x80x2, B and Bxe2x80x2 denote one or more cations from the series
A: Mg2+, Ca2+, Sr2+, Ba2+Zn2+;
Axe2x80x2: Ln3+ (=rare earth element), Bi3+, Al3+, Fe3+;
B: V5+, Nb5+, Ta5+, Mo5+, W5+;
Bxe2x80x2: Ti4+, Zr4+, Hf4+, Sn4+, Ge4+, Si4+, Nb4+, Ta4+ and
x and y denote a number from 0 to 2.
c) oxidenitrides crystallised in the spinel structure of the general formulae
C D2xe2x88x92mDxe2x80x2mO4xe2x88x92m Nm or
C1xe2x88x92n Cxe2x80x2nD2 O4xe2x88x92nNn,
xe2x80x83in which C, Cxe2x80x2, D and Dxe2x80x2 denote one or more cations from the series
C: Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Zn2+;
D: Al3+, Ga3+, In3+, Ti3+, V3+, Cr3+, Fe3+, Co3+, Ni3;
Dxe2x80x2: Ti4+, Zr4+, Hf4+, Sn4+, Ge4+, Si4+, Nb4+, Ta4+;
Cxe2x80x2: Al3+, Ga3+, In3+, Ti3+, V3+, Cr3+, Fe3+, Co3+,
Ni3+ and
m denotes a number greater than 0 to 2 and
n denotes a number from 0 to 1;
d) oxidenitrides crystallised in the elpasolite structure of the general formula
Axe2x80x22Q Bxe2x80x2O5xe2x88x92zN1+z,
xe2x80x83in which z denotes 0, 1 or 2 and, when z=0, Q is a divalent metal ion C, when z=1, Q is a trivalent metal ion Axe2x80x3 and, when z=2, Q is a tetravalent metal ion D, of the formulae
Axe2x80x22 C B O5 N or
Axe2x80x22 Axe2x80x3 B O4 N2 
xe2x80x83in which Axe2x80x2, B, C and D have the above-stated meaning and Axe2x80x3 denotes Ln3+ or Bi3+.
The substances stated in the main claim are suitable as oxidenitride-forming metal compounds. Oxides, mixed oxides, oxide hydrates, carbonates and nitrates are preferred. The metal atoms present in the product are used in the form of one or more educts in the stoichiometric atomic ratio.
According to another embodiment, nitriding proceeds in the rotary tube in the presence of a flux from the series of ammonium salts of carbonic acid, a lower carboxylic acid or a boric acid, boric acids, boric anhydrides, alkali metal halides or alkaline earth metal halides. Fluxes may be added in a quantity of up to 20 wt. %, preferably of 0.5 to 5 wt. %.
Depending upon the selected reaction temperature and the composition of the pulverulent mixture of substances to be nitrided, partial sintering may occur in the rotary tube. It has been found that the duration of nitriding may be shortened and/or pigment quality improved if nitriding proceeds in the rotary tube in two stages; the reaction product from the first stage is ground and then introduced into the second rotary tube stage.
The process according to the invention is very simple to perform. Red to yellow pigments of elevated colouring quality are obtainable on an industrial scale.